Sounds like he got into something he shouldn't have. Like from another animal, prickly bush, posion ivy. It could be many things. I would wash his entire face, head and neck. Apply neosporin or the like, lightly. Contain him somewhere. At least until you can call a vet. If he is an outside dog, check his area and bedding tomorrow. For now he needs to be cleaned, and contained until you find out what's causing it. I will share this with another user who has much more experience. Good luck.